1. Field of the Invention
This invention relates to lasers and, in particular, to the reduction of amplitude and therefore intensity noise in light from lasers, such as semiconductor lasers, configured for detuned operation.
2. Description of the Prior Art
All laser light typically includes both amplitude and phase noise. Phase noise contributes to a finite output spectral linewidth. Amplitude noise limits the performance of lasers in communication systems and other applications.
Amplitude and phase noise in detuned lasers are known to be related. The linewidth enhancement parameter .alpha. has been well studied and provides a measure of the amplitude and phase coupling in lasers resulting from detuned operation, that is, when the linewidth enhancement parameter .alpha. is not equal to zero. It is conventionally believed that semiconductor lasers inherently operated in a detuned fashion.
Linewidth enhancement for semiconductor lasers is known to result at least in part from amplitude fluctuations in the resonant cavity which cause small transient perturbations to the active layer carrier density. Such amplitude fluctuations are known to cause amplitude as well as excess phase noise, that is, linewidth enhancement. The phase noise is a jittering of the instantaneous frequency of the laser resulting from the perturbation in carrier density caused by the amplitude fluctuations because the laser frequency is dependent upon the effective refractive index which is, itself, carrier density dependent.